The present invention relates to a method for forming a joint between a module comprising a chip and a substrate comprising a circuitry pattern in such a manner that an electrical connection is formed between the chip and the circuitry pattern by connecting contact areas of the module and contact areas of the circuitry pattern, and the module is attached to the substrate substantially entirely.
Known adhesive joints are often formed by using one of the following adhesives:                anisotropically conductive adhesive (ACA) is the most common adhesive used for this purpose. It is often used as a paste or a film. ACA requires high temperatures for curing (temperatures around 180° are typical). Also pressure is required when a chip is attached to its substrate because ACA starts to conduct electricity after pressure is applied to the joint. The use of pressure is problematic because during the pressure treatment a continuous process has to be stopped, which limits the capacity of the production line. Typical standstill period may be for example 8 seconds at a time. ACA contains dispersed conductive particles, which size distribution is narrow. The particles have a specific shape and size. The above-mentioned matters make ACA rather expensive.        isotropically conductive adhesive (ICA) typically contains silver particles, which make the adhesive electrically conductive. ICA conducts electricity in all directions, which makes it unsuitable material for joints, which have contact points near each other. A chip can be attached to ICA without using any pressure, which makes possible continuous manufacturing without any standstills. Thus production capacity is good with this adhesive. Further, ICA is much cheaper than ACA.        nonconductive adhesive (NCA) is quite cheap because there are no conductive particles inside the adhesive. The problem related to its use is that the surface to which it is applied requires a pretreatment step because otherwise no proper electrical conductivity through the joint is achieved.        
Further, in connection with methods in which an elongated module comprising a chip is attached to a circuitry pattern, is known a method called crimping. The crimping method comprises a pressure impact, which is directed to the ends of the elongated module in order to electrically connect the module and the circuitry pattern.
Publication WO 02/49093 discloses a method for attaching a module comprising a chip to a substrate comprising a circuitry pattern. The module is adhered to the substrate by using a thermoplastic material, which can be either anisotropically conductive or nonconductive.
Publication WO 03/56509 also discloses a method for attaching a module comprising a chip to a substrate comprising a circuitry pattern. The module is adhered to the substrate by using a thermoplastic film. An electrical contact between a contact point of the circuitry pattern and a contact point of the chip is obtained by pressing the contact points together/close enough mechanically.
In spite of advantageous effects achieved by the above-mentioned methods for attaching modules, several drawbacks still exist. Anisotropically conductive thermoplastic material is quite expensive. Nonconductive thermoplastic material is cheaper but electrical contacts formed between the chip and the circuitry pattern are unreliable. The combination of a thermoplastic film and mechanical pressing leads to a reliable joint but as with all the prior methods described above, excess process steps are required for the attachment of the thermoplastic material prior to that process step in which the module is adhered to the substrate.